Many portable electronic devices such as cameras, cellular telephones, personal digital assistants (PDAs), MP3 players, computers and other devices include an imager for capturing images. One example of an imager is a complementary metal-oxide semiconductor (“CMOS”) imager. A CMOS imager includes a focal plane array of pixels, each one of the pixels including at least one photosensor overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate. Each pixel may include at least one electronic device such as a transistor for transferring charge from the photosensor to a storage region.
Each pixel has corresponding readout circuitry that includes at least a charge storage region connected to the gate of an output transistor, an output source follower transistor, a reset transistor for resetting the charge storage region to a predetermined charge level, and a row control transistor for selectively connecting the readout circuitry to a column line. The charge storage region may be constructed as a floating diffusion region. Each pixel may have independent readout circuitry, or may employ common element pixel architecture (CEPA), that may include multiple pixels sharing the same readout circuitry.
In a CMOS imager, the active elements of a pixel circuit perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region accompanied by charge amplification; (5) selection of a pixel circuit for readout; and (6) output and amplification of a signal representing a reset level and pixel charge. Photo charge may be amplified when the charge moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
Image sensors have a characteristic dynamic range. Dynamic range refers to the range of incident light that can be accommodated by an image sensor in a single frame of pixel signal values. It is desirable to have an image sensor with a high-dynamic range in order to image scenes that generate high-dynamic range incident signals, such as indoor rooms with windows to the outside, outdoor scenes with mixed shadows and bright sunshine, night-time scenes combining artificial lighting and shadows, and many others.
The dynamic range for an image sensor is commonly defined as the ratio of its largest non-saturating signal to the standard deviation of its noise under dark conditions. The dynamic range is limited on an upper end by the charge saturation level of the sensor, and on a lower end by noise imposed limitations and/or quantization limits of the analog-to-digital converter used to produce the digital image. When the dynamic range of an image sensor is too small to accommodate the variations in light intensities of the imaged scene e.g., by having a low saturation level, image distortion may occur.
A common method of high-dynamic range imaging is the multiple exposure capture method, wherein several images are captured at different integration periods and are then combined to create a high-dynamic range image. With multiple exposure image capture (and in any high-dynamic range imaging system using sequential exposures including lateral overflow methods), a moving object will be registered at different pixel positions in each exposure. The registration discrepancy is most severe with frame-sequential multiple exposure capture, but can also be significant in row-sequential multiple exposure capture, such as may be used in CMOS imagers. If one of the integration periods is long relative to the scene motion, the object shape will appear blurred and elongated in the direction of motion in this exposure.
When the multiple images are combined using a basic multiple exposure combination method, the discrepancy in position and shape of the moving object in the multiple exposures will result in misregistration of the object in the combined image. This may cause image artifacts such as a bright outline and/or a noisy region (see, e.g., FIG. 3, described in more detail below). These image artifacts degrade the image and are undesirable.
Accordingly, there is a desire for a method and apparatus which is able to identify and remove the image artifacts caused by motion in multiple-exposure high-dynamic range imaging.